It has become fairly common practice in the design of electronic tone generators to employ resistor-capacitor circuits, or networks, (RC networks) to amplitude modulate the tone signals. The rapid charge of a capacitor and its subsequent discharge through a resistor produces a decaying voltage waveform similar to tones produced by musical instruments.
U.S. Pat. No. 4,273,019, granted June 16, 1981, to M. Goto for "Electronic Tone Generator" and No. 4,545,279, granted Oct. 8, 1985, to T. Sano for "Electronic Music Note Generator", disclose tone generators employing RC networks for shaping tone waveforms.
In theory, the waveform generated by the RC network in the generators disclosed in the above-mentioned patents can be predetermined by proper selection of values for the capacitor and the resistor and by control of the value and duration of the voltage applied to the network. In practice discharge rate of the capacitor is influenced by the impedence of components to which the output of the RC network is applied, such as an electroacoustic output device and an amplifier therefor. Unless the impedence of these other components is kept high, leakage from the capacitor through these other components alters the waveform intended to be produced by the RC network. This in turn results in the generation of sound different than that intended.
In the past it has been the practice to design and fabricate the output portions, i.e. the electroacoustic device and its amplifier, to exhibit high impedence by, for example, employing a Darlington pair transistor circuit as an amplifier. The disadvantages of doing this are that it increases the cost of the generator circuit and requires that a higher operating voltage be employed.
The problem of waveform distortion from capacitor leakage has been even more acute with tone generators having a variable resistor in the circuit for controlling the volume of tones emitted by the electroacoustic device. Obviously, adjusting the variable resistor changes the impedance of the load on the RC network and this adversely affects the waveform signal from the RC network.